112 research outputs found
The Minispiral in the Galactic Center revisited
We present the results of a re-examination of a [Ne II] line emission data
cube (\lambda 12.8 \mu m) and discuss the kinematic structure of the inner \sim
3 \times 4 pc of the Galaxy. The quality of [Ne II] as a tracer of ionized gas
is examined by comparing it to radio data. A three dimensional representation
of the data cube allows us to disentangle features which are projected onto the
same location on the sky. A model of gas streams in different planes is fitted
to the data. We find that most of the material is located in a main plane which
itself is defined by the inner edge of the Circum-Nuclear Disk in the Galactic
Center. Finally, we present a possible three dimensional model of the gas
streams.Comment: 12 pages, 18 figures; submitted to New Astronomy; higher resolution
version and two animations available via anonymous ftp
ftp://ftp.ita.uni-heidelberg.de/pub/ITA/wjd/Minispira
Torus models for obscuration in type 2 AGN
We discuss a clumpy model of obscuring dusty tori around AGN. Cloud-cloud
collisions lead to an effective viscosity and a geometrically thick accretion
disk, which has the required properties of a torus.
Accretion in the combined gravitational potential of central black hole and
stellar cluster generates free energy, which is dissipated in collisions, and
maintains the thickness of the torus. A quantitative treatment for the torus in
the prototypical Seyfert 2 nucleus of NGC 1068 together with a radiative
transfer calculation for NIR re-emission from the torus is presented.Comment: 6 pages, 2 figures, contributed paper to Proceedings of the
Conference "Growing Black Holes" held in Garching, Germany, June 21-25, 2004,
edited by A. Merloni, S. Nayakshin and R. Sunyaev, Springer-Verlag series of
"ESO Astrophysics Symposia
VIVA, VLA Imaging of Virgo spirals in Atomic gas: I. The Atlas & The HI Properties
We present the result of a new VLA HI Imaging survey of Virgo galaxies, VIVA
(the VLA Imaging survey of Virgo galaxies in Atomic gas). The survey includes
high resolution HI data of 53 carefully selected late type galaxies (48 spirals
and 5 irregular systems). The goal is to study environmental effects on HI gas
properties of cluster galaxies to understand which physical mechanisms affect
galaxy evolution in different density regions, and to establish how far out the
impact of the cluster reaches. As a dynamically young cluster, Virgo contains
examples of galaxies experiencing a variety of environmental effects. Its
nearness allows us to study each galaxy in great detail. We have selected Virgo
galaxies with a range of star formation properties in low to high density
regions (at the projected distance from M87, d_87=0.3-3.3 Mpc). Contrary to pr
evious studies, more than half of the galaxies in the sample (~60%) are fainter
than 12 mag in B_T. Overall, the selected galaxies represent the late type
Virgo galaxies (S0/a to Sd/Irr) down to m_p<~14.6 fairly well in morphological
type, systemic velocity, subcluster membership, HI mass and deficiency. In this
paper (VIVA I: the atlas and the HI properties), we present HI maps and
properties, and describe the HI morphology and kinematics of individual
galaxies in detail (abbreviated).Comment: K band magnitudes for 6 galaxies in Table 3 have been corrected. One
of the labels in Figure 8 is corrected and an omission in the acknowledgments
has been added. The latter two were correct in the previous astro-ph version
but are wrong in the journal version. A full resolution with the complete HI
atlas can be downloaded at http://www.astro.yale.edu/viva/pub.htm
Virgo Galaxies with Long One-Sided HI Tails
In a new HI imaging survey of Virgo galaxies (VIVA: VLA Imaging of Virgo
galaxies in Atomic gas), we find 7 spiral galaxies with long HI tails. The
morphology varies but all the tails are extended well beyond the optical radii
on one side. These galaxies are found in intermediate-low density regions
(0.6-1 Mpc in projection from M87). The tails are all pointing roughly away
from M87, suggesting that these tails may have been created by a global cluster
mechanism. While the tidal effects of the cluster potential are too small, a
rough estimate suggests that simple ram-pressure stripping indeed could have
formed the tails in all but two cases. At least three systems show HI
truncation to within the stellar disk, providing evidence for a gas-gas
interaction. Although most of these galaxies do not appear disturbed optically,
some have close neighbors, suggesting that tidal interactions may have moved
gas outwards making it more susceptible to the ICM ram-pressure or viscosity.
Indeed, a simulation study of one of the tail galaxies, NGC 4654, suggests that
the galaxy is most likely affected by the combined effect of a gravitational
interaction and ram-pressure stripping. We conclude that these one-sided HI
tail galaxies have recently arrived in the cluster, falling in on highly radial
orbits. It appears that galaxies begin to lose their gas already at
intermediate distances from the cluster center through ram-pressure or
turbulent viscous stripping and tidal interactions with neighbours, or a
combination of both.Comment: 4 pages, 3 figures (including 1 plate), accepted for accepted for
publication in ApJ Letter (vol. 659, L115), a version with full resolution
Plate 1 is available from
http://www.astro.umass.edu/~achung/astro-ph/viva_tail.pd
CHANG-ES VII: Magnetic outflows from the Virgo cluster galaxy NGC 4388
We investigate the effects of ram pressure on the ordered magnetic field of a
galaxy hosting a radio halo and strong nuclear outflows. New radio images in
total and polarized intensity of the edge-on Virgo galaxy NGC\,4388 were
obtained within the CHANG-ES EVLA project. The unprecedented noise level
reached allows us to detect striking new features of the ordered magnetic
field. The nuclear outflow extends far into the halo to about 5\,kpc from the
center and is spatially correlated with the and X-ray emission.
For the first time, the southern outflow is detected. Above and below both
spiral arms we find extended blobs of polarized emission with an ordered field
oriented perpendicular to the disk. The synchrotron lifetime of the cosmic ray
electrons (CREs) in these regions yields a mean outflow velocity of
(270\pm70)\kms, in agreement with a galactic wind scenario. The observed
symmetry of the polarized halo features in NGC 4388 excludes a compression of
the halo gas by the ram pressure of the intra-cluster medium (ICM). The
assumption of equilibrium between the halo pressure and the ICM ram pressure
yields an estimate of the ICM density that is consistent with both the ICM
density derived from X-ray observations and the recent \textit{Planck}
Sunyaev-Zel'dovich measurements. The detection of a faint radio halo around
cluster galaxies could thus be used for an estimate of ICM ram pressure
Dense Cloud Ablation and Ram Pressure Stripping of the Virgo Spiral NGC 4402
We present optical, HI and radio continuum observations of the highly
inclined Virgo Cluster Sc galaxy NGC 4402, which show evidence for ram-pressure
stripping and dense cloud ablation. VLA HI and radio continuum maps show a
truncated gas disk and emission to the northwest of the main disk emission. In
particular, the radio continuum emission is asymmetrically extended to the
north and skewed to the west. The Halpha image shows numerous HII complexes
along the southern edge of the gas disk, possibly indicating star formation
triggered by the ICM pressure. BVR images at 0.5" resolution obtained with the
WIYN Tip-Tilt Imager show a remarkable dust lane morphology: at half the
optical radius, the dust lane of the galaxy curves up and out of the disk,
matching the HI morphology. Large dust plumes extend upward for ~1.5 kpc from
luminous young star clusters at the SE edge of the truncated gas disk. These
star clusters are very blue, indicating very little dust reddening, which
suggests dust blown away by an ICM wind at the leading edge of the interaction.
To the south of the main ridge of interstellar material, where the galaxy is
relatively clean of gas and dust, we have discovered 1 kpc long linear dust
filaments with a position angle that matches the extraplanar radio continuum
tail; we interpret this angle as the projected ICM wind direction. One of the
observed dust filaments has an HII region at its head. We interpret these dust
filaments as large, dense clouds which were initially left behind as the
low-density ISM is stripped, but are then ablated by the ICM wind. These
results provide striking new evidence on the fate of molecular clouds in
stripped cluster galaxies.Comment: 17 pages, 4 figures, accepted for publication in AJ. See
ftp://ftp.astro.yale.edu/pub/hugh/papers/crowl_n4402.ps.gz for a version with
high-resolution figure
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